The use of metallic oxide semiconductor sensors for the detection of various gases and vapors is well known. For many years, a Japanese company, Figaro Engineering Co., Inc. of Osaka, Japan, has been manufacturing and marketing a family of such sensors based upon Tin Oxide for gas detection as described in U.S. Pat. No. 3,676,820. In practice the resistance of the tin oxide is measured, usually while it is heated. The resistance changes dramatically when even small amounts of organic vapors, Carbon Monoxide, or even water vapor are present.
The utility of these sensors for use as Carbon Monoxide detectors is frequently compromsied by their sensitivity to these other gases, by changes of the sensor resistance due to ambient temperature and humidity and by the tendency of the sensor to saturate with exposure even to low concentrations of gas if such levels persist for a long enough time.
Recently a method has been disclosed by Le Vine, U.S. Pat. No. 3,906,473 which enhances the sensitivity to CO of these sensors , while reducing responses from undesired organic vapors and reducing the possibility of saturation. The method consists of operating the sensor in a cyclic manner wherein the tin oxide sensor is heated to a high temperature for a short period of time, followed by another period at a lower temperature. The resistance of the tin oxide sample is measured at the end of the lower temperature period. The theory as disclosed in the patent is that the high temperature purges the sensor, driving off the volatile components and returning the sensor to its high impedance state.Then, when the sensor cools to the lower temperature, where it is more sensitive to CO, a more accurate measurement of CO can be made. Since the sensor is continuously cycled between the two temperatures, saturation of the sensor is prevented. While the Le Vine method is an improvement in Tin Oxide sensor operation, compensation still must be made for changes in ambient temperature, usually by means of a thermistor matched to the temperature coefficient of tin oxide. Additionally the tin oxide is subject to changes in response if the humidity changes, requiring humidity compensation which is neither simple nor inexpensive.
Such sensitivity problems have limited the application of these sensors. The possibility of false alarms prevents the use of such sensors in critical applications such as automatic shutoff devices for combustion appliances.
The present invention is a novel improvement in the method of Le Vine as discussed above. This improvement compensates for response changes due to changes in ambient temperature and humidity, allowing operation from below 15% to above 90% relative humidity and from below 0 degrees C. to above 50 degrees C. without the need for additional compensating elements. This improvement also increases the accuracy of the CO measurement, allowing these inexpensive sensors to be used for quantitative measurement of CO concentration. Inexpensive devices can now be built which measure the time weighted average of CO concentration. Such devices much more accurately model the physiological effects of CO and thus can more easily determine when the CO dose has reached a dangerous level. Present devices to perform this function are based on expensive and short lived electrochemical sensors, but since they can now be made with inexpensive tin oxide sensors using this novel method, very low priced CO alarms can be easily produced. Safety shutoff systems for combustion devices can now be built using this novel method to provide a very high degree of freedom from false alarms.